A new family with NTHL1-associated polyposis (NAP) is described, involving a 58-year-old male affected with >100 colorectal polyps and a 55-year-old female sibling with nine colorectal polyps. The female was also diagnosed with a thyroid adenoma at age 40. Significantly, no malignant neoplasms have been detected in this family, which is important to further delineate the clinical phenotype related to NAP. A review of previously reported obligate heterozygous carriers of NTHL1 variants showed two patients affected with neoplasms at <55 years of age, generating a study to outline the phenotypic spectrum in patients with heterozygous pathogenic NTHL1 variants relevant.